1. Field of the Invention
This invention relates to a method and arrangement for prediction coding via correlation between consecutive frames of digital image signals for data compression or redundancy reduction of information to be transmitted, and more specifically to such a method and arrangement which feature high reproduction quality ranging from still pictures to very rapidly moving ones.
2. Description of the Prior Art
In order to compress digital images to be transmitted for the purposes of redundancy reduction, interframe prediction through correlation has been found effective. This prior art technique, when applied to still or relatively still images or pictures, is capable of realizing very high data compression as well as high reproduction quality.
The above-mentioned known technique, however, has encountered the problem that, when encoding images exhibiting rapid movement or displacement while maintaining high data compression, the reproduced pictures tend to degrade in quality. This is because the interframe correlation decreases with increase movement of an object between frames. A known approach to solving this problem is "motion compensation" technique, which utilizes intraframe movement or displacement (motion vector) to obtain high prediction probability. Motion compensation has found excellent applications in encoding image or picture signals of objects which are undergoing rapid movement, and hence high data compression as well as high fidelity of picture reproduction can be achieved. For further data relating to this motion compensation technique, reference should be had to an article entitled "Television Bandwidth Compression Transmission by Motion-Compensated Interframe Coding" (IEEE Communication Magazine, pp. 24-30, November 1982), and also to U.S. Pat. No. 4,077,053 issued on Feb. 28, 1978 in the name of Ishiguro.
The motion compensation, however, is not suitable in applying same to encoding signals of objects which are undergoing extremely rapid movement. More specifically, there exists the problem that the amount of encoded information becomes excessive due to inappropriate data compression and cannot be transmitted at the limited transmission rates available. In order to overcome the problem, viz., to reduce the information to be transmitted, the following approaches have been proposed: (a) changing the quantization characteristics from fine to coarse by increasing the quantization level, (b) reducing the number of pixels to be encoded (viz., subsampling), (c) reducing the number of fields to be encoded (viz., frame skipping), and (d) temporarily terminating the encoding itself. However, utilizing coarse quantization characteristics leads to impairment of reproduced pictures called "dirty window effect". That is, the reproduced pictures become dim as if viewed through a soiled window. On the other hand, the subsampling or frame skipping may cause temporary stoppage of reproduced pictures. This phenomenon tends to disturb the viewer rather than obscuring the reproduced pictures due to lowering of spatial resolution.
In an effort to realize effective redundancy reduction in transmitting very rapid image signals, it has been proposed to apply orthogonal transformation to the prediction error signals derived from motion compensation. In connection with this prior art, reference should be had to "Displacement Measurement and Its Application in Interframe Image Coding" (IEEE Trans. Commun., Vol. COM-29 No. 12, pp. 1799-1808, December 1981). When applying this prior art to the case where encoded information is transmitted at a low transmission rate, most of the AC (alternate current) components of the orthogonal transformation coefficients should be neglected. In this case, it is usual to cut off or discard the coefficients corresponding to sophisticated patterns. Consequently, the reproduced complex pattern becomes somewhat dim. This however, causes no significant problem concerning visual sensation in that the viewer is not particularly sensitive to the obscurity of a very rapidly moving image.
It is understood that the orthogonal transformation contributes to effective data redundancy reduction if applied to encoding of pictures wherein the image is moving very rapidly. This stems from the fact that the correlation between consecutive frames is very high.
Contrarily, when encoding slow moving image information, the orthogonal transformation is no longer useful in that the interframe correlation is low. This can be illustrated by encoding the signals of slow or not extremely rapid moving images using the motion compensation, in the case of which the prediction errors occur isolately near the boundaries of a moving object. This means that the intraframe correlation of the prediction error signals is very low. As a result, if the orthogonal transformation is used in the above case, the reproduced picture quality is impaired to a considerable extent.
According to the prior art, however, there is no method or arrangement which enables the encoding of images ranging from still pictures to very rapidly moving pictures while maintaining high data compression as well as high reproduction quality.